Relative Flux Calibration

Were the spectra taken at a similar air mass? The instrument response includes the effect of the atmosphere as well as the instrument. 

Regards Andrew

John,

I find the freehand drawing of the response curve can produce markedly different results with different attempts.

Also, the placing of the star on/in the slot (and what portion is recorded) also plays it’s part.

Don’t get paranoid about your setup – yet.

Keep on swimming. 

Regards

Tony

I’ve experienced the same problem as John with the same sequence of Miles spectra which Robin used. The response profiles calculated for each star give an excellent match with the Miles spectrum for that star but differ from star to star. All the spectra were taken over a period of an hour at an air mass of around 1.2 with no change to the equipment including focus and no obvious change in atmospheric conditions. However, I do live about 50 miles west of Heathrow and planes are flying over at high altitude all the time. I recently asked on the ARAS Forum if anyone have noticed changes in atmospheric response due to aircraft con trails but so far there has been no response. Possibly a bit of a long shot.

I repeated the exercise a few days later and found the same variation, again different response profiles from star to star but no consistency with the first set. I now plan to experiment with varying the position of the guide star relative to the slit for the same star to see what effect this has. That will not be for a while as the forecast here is bad for several days.

I’m using a LISA with a 23 micron slit on a C11 with a focal reducer operating at f/5.5. The slit size on the sky is 3.1″ which is a reasonable match to the seeing here. These is a noticeable fishtail on the spectra due to a combination of chromatic aberration in the LISA and the FR lens but I include the full fishtail that in the binning zone and do this with the same binning zone for all the stars. I use the optimal binning option in ISIS but switching that off has no effect on the derived response profiles. It is not a problem with smoothing the response profile as the effect is much larger than any variation that causes.

Cheers, David

Hi Andrew,

A wide “photometric” slit is certainly needed for absolute flux calibration of course but in my experience, provided precautions are taken I have found it is possible to consistently get “reasonable” (+-10% say above 4000A) broad continuum relative flux calibration with my setup using a slit width the same order as the star PSF.

(Except for the effect of changing focus on the original run and an example of the effects of scintillation on the latest run, the continuum errors my two runs using MILES stars I have documented here have been reasonably under control. You can also see some comparisons with professional spectra in what is perhaps a worse case scenario of faint targets using the modified ALPY 200 here.

http://www.threehillsobservatory.co.uk/astro/spectroscopy_20.htm

Comparisons between observers on the same object posted on the ARAS forum using the reference star techniques are also usually (but with admittedly some exceptions) consistent. 

Perhaps my success though is due to the not particularly good guiding performance which effectively scans the centre of the star image across the slit.  

Speaking to a professional recently, this sort of thing is tough for them too and their approach seemed to be to specifically measure the instrument response using standard stars only occasionally on photometric nights, removing the effect of the atmosphere by measuring at different air masses and then measure just the atmospheric extinction during the observing session, typically making one standard star measurement and correcting for air mass using an atmospheric model generated for their particular local conditions. This gets round the problem of having to find a reference star close to the target but does not address the problem of selective wavelength  sampling at the slit which may be what we are seeing here, though I believe that is yet to be proved. (David, perhaps you could try running with a wider 35um or even 50um slit to see if you get any improvement?).

Buil’s ideas on spectrophotometric measurements using a combination of wide and narrow slits are probably worth thinking about in this context too, though adopting the 4 spectra measurement every time would reduce throughput significantly and might still give extinction errors with our fickle weather.

http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm

Professionals though probably measure with the slit orientated at the parallactic angle and their CA and positioning and guiding errors are likely much lower than ours in general so perhaps selective sampling at the slit is not a big issue for them 

Cheers

Robin

I don’t disagree with what you say Robin. If you have a technique that delivers the accuracy you need for your objectives e.g. you SN matching work then absolutely fine. The difficulty I have is when corrected spectra or even flux calibrated spectra are presented with no estimate of their errors in the continuum level. (I don’t think the BeSS standard requires it.)

If you are using your own data I am a great believer in doing the minimum processing consistent with your goals to minimise added noise and potential artifacts. As we have commented before it is different when adding data to a general use database.

Regards Andrew

Hi Paul,

Yes it is described in part 4 of the link I posted above.

http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm

It seems to be what the pro I talked to was suggesting too. ie separating out the instrument response (which is presumed stable so measured infrequently) and extinction (measured on the night and adjusted for air mass using the AOD)

Cheers

Robin

Yes, I believe so Robin. In fact that’s exactly what I did for the aforementioned high cadence campaigns (gam2 Vel, zet Pup). I created a response correction, initially processing the reference star with AOD enabled. At the time I assumed it was necessary in order to compare to the (presumably corrected) published references when creating the IR profile.

I must confess though, I’m no expert. Bernard H., provided much of the guidance during my initiation into these campaigns, having spent many more hours (years !) working with both the ISIS and the pros in question. I’m still [mostly] confused about how to use this feature, particularly when it comes to determining the correction value. As such I’ve reverted to back the conventional method of correction using a nearby standard for each session and leave AOD switched off.

I do recall seeing some analysis (again, from Bernard) that showed only minor – almost insignificant – variations between the two methods – supporting the consistency of the ‘one IR’ method.

Cheers,

Paul

I’d played around with AOD having gained an idea of what it might be from here…

https://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MODAL2_M_AER_OD

Experimenting with varying target altitudes obviously give quite serious differences but using differing AOD values between 01 – 0.2 (the UK apparent variances) produced huge differences in the Aerosol component.  It seems to vary considerably from week to week (?).

I’m not sure how ISIS applies this (?) or indeed whether UK values are accounted for.  (They certainly appear different from the guides given in ISIS).

Or am I reading this wrong?

Regards

T

Hi Andy, all,

The response tool in ISIS divides the spectrum of our processed standard star by a Miles (or other) standard to create a response profile – which is then smoothed before applying to our target spectra during processing.

Assuming the database standard has been corrected for atmosphere, shouldn’t we be doing the same (ie, invoking ‘auto atmosphere’ with an AOD value) when we pre-process our standard star? Otherwise, how does ISIS correctly correlate the atmosphere corrected Miles standard with our non-atmosphere corrected calibration star.

What am I missing?

Paul